The present invention relates to an active matrix type display device and a display method thereof. The active matrix type display device means a display device in which pixels are arranged at respective intersecting points of a matrix, every pixel is provided with a switching element, and image information is controlled by on/off switching of the switching elements. Examples of display media for the active matrix type display device are a liquid crystal, plasma and other bodies or states whose optical characteristic (reflectance, refractive index, transmittance, light emission intensity, or the like) can be changed electrically. The invention particularly relates to an active matrix type display device which uses, as the switching element, a three-terminal element, i.e., a field-effect transistor having the gate, source and drain.
In describing the invention, the term "row" of a matrix means a structure in which a signal line (gate line) that is disposed parallel with a row concerned is connected to the gate electrodes of transistors belonging to the row. The term "column" means a structure in which a signal line (source line) disposed parallel with a column concerned is connected to the sources (or drains) of transistors belonging to the column. A circuit for driving the gate lines and a circuit for driving the source lines are called a gate driver and a source driver, respectively.
Flat panel displays (FPDs) have been developed as new display devices to replace a CRT display. The active matrix type display device is typical of those flat panel displays. In the active matrix type display device, a screen is divided into pixels and the individual pixels are provided with respective switching elements, which control display information that is retained by the pixels. A typical example of the active matrix type display device is a thin-film transistor (TFT) active matrix display using a TN (twisted nematic) liquid crystal.
In this display device, the display medium is the TN liquid crystal and the image information is voltages of the pixels. That is, the transmittance of the TN liquid crystal (display medium) is controlled by a voltage retained by each pixel. Conventionally, in this type of active matrix type display device, an image is rewritten by updating display contents of all the pixels by top-to-bottom sequential scanning of rows. The image rewriting is performed at a frequency of every frame, i.e., 30 to 60 times per second (30-60 Hz).
However, for certain types of display contents, the image rewriting of such a frequency is not always necessary. For example, a still image need not be rewritten until voltages retained by the pixels decrease to such low values as cannot provide sufficient display quality. Even in the case of moving images, not all the pixels display different image information every time.
The image rewriting requires output of signals, which is a factor of increasing power consumption and, therefore, an obstacle to portable applications.